Manual actuators for automatic control valves of the type in which the valve element is moved linearly generally incorporate a power screw rotated by a handwheel. In applications where relatively high force is required to actuate the valve element, a worm gear set between the handwheel and the power screw is often required to multiply the torque applied to the valve element.
The power screw is usually mounted above the automatic actuator and in line with the valve stem and actuator shaft, the power screw pushing or pulling on the actuator shaft which operates the valve stem.
In a valve operator of this type, it is advantageous if the manual actuator is self-locking so that unbalanced loads on the valve element cannot change the valve position when the operator is in its manual mode of operation. Since the valve element must be free to move when in the automatic mode of operation, it is necessary to stroke the manual actuator of a self-locking unit into a retracted position when valve operation is transferred from the manual to the automatic mode of operation. Also, when it is desired to transfer from the automatic mode to the manual mode, it is necessary to stroke the manual actuator until its position coincides with that of the valve before manual actuation of the valve can begin. This requirement of stroking the manual actuator when transferring from one mode to the other mode of operation is time and energy consuming and can put a practical limitation on the length of stroke of the valve element which this type of operator can accommodate and causes delays relative to emergency operation.
Because of the foregoing, it has become desirable to develop a valve operator which can be easily transferred from the manual to the automatic mode of operation and vice versa without stroking the manual actuator, and which is of simple construction.